1. Field of the Invention
The present invention relates to an antenna device and a wireless device including the same.
2. Description of the Related Art
The radio frequency identification (RFID) system, in which a reader/writer and an RFID tag communicate with each other in a non-contact manner to transmit information between the reader/writer and the RFID tag, has become widespread. Such an RFID tag and a reader/writer include a radio frequency integrated circuit (RFIC) chip that processes high frequency signals and an antenna element that emits or receives high frequency signals. Examples of known RFID systems include the HF-band RFID system using the 13 MHz band, such as FeliCa®, and the UHF-band RFID system using the 900 MHz band which is used in warehouse management and the like.
A general RFID tag often adopts bonding that uses a gold bump for connecting an RFIC chip to an antenna element. This type of bonding, however, uses ultrasonic waves and therefore the bonding process is complicated. As a result, high connection reliability may not be ensured.
Accordingly, techniques disclosed in Japanese Patent Registration No. 2834584, Japanese Unexamined Patent Application Publication No. 2008-211572, and Japanese Unexamined Patent Application Publication No. 2009-111950 are known in which a feed coil is connected to an RFIC chip and the feed coil is connected to a main antenna via an electromagnetic field.
In an RFID tag in which a feed coil is connected to an RFIC chip and the feed coil is coupled to a main antenna via an electromagnetic field, connection between the RFIC chip and an antenna element can be readily made, however, this coupling is negligibly enhanced and, when a signal or electromagnetic energy received by the main antenna is transmitted to the RFIC chip, insertion loss may increase. As a result, the maximum communication distance may not be attained.
This is not only a problem for an RFID tag but is a problem also for an antenna device to which a feeder circuit including an RFIC is connected and a wireless device including the same.